I know that Metro on both platforms shares a common “look and feel” and that WinRt makes use of XAMLand C# (or VB.NET) like Silverlight does on Windows Phone 7.
Howev
The biggest potential problem I experienced is if you happen to have chosen to develop with XNA/Silverlight on the Windows Phone. In that case you either have to decide to completely rewrite the application or try out an XNA replacement such as MonoGame. Having gone that route myself I can happily say that, at least at this stage, MonoGame seems to support at least 90% of the functionality of XNA and due to the hard work of a few groups of developers and individuals it is now relatively easy to port over the application without too many fundamental changes.
Here is the link to MonoGames website http://monogame.codeplex.com/